1. Field of the Invention
The present invention relates to an ink suitable for ink-jet recording, and an ink-jet recording method employing the ink. More specifically, the present invention relates to an ink which realizes high-quality recording with excellent fixation of the ink on plain paper such as wood-free paper, copying paper, letter paper, heat-transfer paper, business form paper for wire-dot printing, and also to an ink-jet recording method employing the above ink.
The present invention also relates to an apparatus charged with the above ink.
2. Related Background Art
Conventionally for ink-jet recording, an aqueous ink is used which comprises a water-soluble dye and an aqueous medium for dissolving the dye. The ink for ink-jet recording is required:
(1) to give sufficient density of an image,
(2) to dry quickly on a recording medium,
(3) to cause less bleeding of recorded images,
(4) not to cause flow-out even when the image is brought into contact with water or alcohol,
(5) to form recorded images having sufficient light-fastness,
(6) not to clog an ink nozzle,
(7) not to cause inconvenience such as blurring of a recorded image during continuous recording or after interruption of recording for a long time,
(8) to be stable in storage, and
(9) to be safe for persons handling the ink,
In ink-jet recording utilizing thermal energy, the ink is further required:
(10) to have sufficient thermal stability, and to give no adverse effect to a thermal energy-generating means.
These requirements cannot all be satisfied simultaneously with conventional material constitution of ink-jet recording inks. Therefore, conventional inks do not necessarily satisfy the recent requirements for the performance of inks.
Most conventional inks, which are composed of a water-soluble dye, water, and a water-soluble organic solvent, involve a particularly difficult problem of inconsistency of printed image quality with ink fixability. In recent years, solid jet technique has been put to practical use. In this technique, a solid ink is liquefied at ejection, and the ink medium itself solidifies after the ejection, thereby achieving extremely quick fixation of ink. Therefore this technique gives high quality of recorded images independently of the kind of recording paper used. The solid ink, however, has the disadvantages that the life of a recording head is adversely affected by required head temperatures as high as 100xc2x0 C.; much power is consumed for maintaining the head temperature; and the apparatus is necessarily large in size. Furthermore, the practical use of the solid ink is restricted because the viscosity of the liquefied ink has a lower limit even at a higher heating temperature, which leads to lower frequency responsiveness.
On the other hand, an ink for ink-jet recording is proposed which contains less or no water, and employs a low-viscosity organic solvent as a medium for dissolving or dispersing a dye. Such a solvent includes alkyl ethers such as ethylene glycols, esters, pyrrolidones, lactones, and cyclic carbonates. The recording liquid which employs such an organic solvent as the medium permeates rapidly into plain paper and naturally exhibits high fixability. In order to prevent running and strike-through of the ink and to keep the printed image quality by adjustment of the ink formulation, measures are taken such that a solid component like a wax and a polymeric compound is added to the ink with the dye component, or that a binder component is added together with the pigment-dispersing resin.
In such methods, it is important that the rate of drying by evaporation and permeation of the constituting medium after ink deposition on a recording medium needs to be well balanced with the rate of increase of ink viscosity in order to obtain satisfactory fixability and image quality. In most cases, however, the ink fixability is low or the image quality is low with such measures.
In any of the above cases, addition of a binder component disadvantageously increases ink viscosity to lower the response frequency of ejection, or lowers responsiveness of ejection at the start of the recording owing to drying and viscosity increase of the ink at the tip of the nozzle. Therefore the addition of a binder is considered to be impractical.
The present invention intends to provide an aqueous ink which is satisfactory both in ink fixability and printed image quality, and to solve the aforementioned problems that are not solved by addition of a known water-soluble resin of prior art.
To achieve the above object, the present invention provides an ink containing a coloring material and an aqueous liquid medium for solving or dispersing the coloring material, the liquid medium having a lower critical consolute temperature (Tc) to cause phase separation at a temperature in the range of from 40 to 100xc2x0 C.
The ink of the present invention contains preferably a polymeric compound giving a lower critical consolute temperature in the liquid medium, the polymeric compound being a copolymer having as a monomer unit an N-alkyl-substituted acrylamide. Otherwise, the liquid medium contains a solvent which is preferably a compound giving a lower critical consolute temperature.
The present invention provides further an ink-jet recording method which conducts recording by ejecting ink droplets through an orifice of a recording head onto a recording medium by use of the aforementioned ink.
The ink-jet recording is practiced in the present invention as follows:
1) an ink is ejected in the form of liquid droplets by action of thermal energy provided to the ink, and 2) at the time of recording, the recording medium is kept at a temperature higher than the lower critical consolute temperature of the liquid medium of the ink, or otherwise 3) at the time of recording, the recording head is kept at a temperature lower than the lower critical consolute temperature of the liquid medium of the ink, and the recording medium is kept at a temperature higher than the lower critical consolute temperature.
The present invention further provides a recording unit having an ink container for holding an ink, and a head for ejecting the ink as ink droplets, the ink being the aforementioned one of the present invention. The head of the recording unit ejects ink by action of thermal energy given to the ink.
The present invention further provides an ink cartridge having an ink container for holding ink, the ink being the aforementioned ink of the present invention.
The present invention further provides an ink-jet recording apparatus having an ink container for holding an ink, and a recording unit having a head for ejecting the ink as ink droplets, the ink being the aforementioned one of the present invention. The recording apparatus has a head for ejecting the ink as liquid droplets, and a means for heating a recording medium.
The present invention still further provides an ink-jet recording apparatus having a recording unit for ejecting droplets of an ink, an ink cartridge having an ink container for holding the ink, an ink supplying device for supplying the ink from the ink cartridge to the recording unit, the ink being the aforementioned ink of the present invention. The recording unit has a head for ejecting the ink as liquid droplets, and a means for heating a recording medium.